Patent ES 2104708 discloses a class of compounds with special agonism through the 5-HT1-like receptors and excellent absorption following oral administration. These properties make the compounds particularly useful in the treatment of migraine, cluster headache and headache associated with vascular disorders. One of the preferred compounds of that patent is (S)-4-[3-(2-dimethylaminoethyl)-1H-indol-5-ylmethyl]-1,3-oxazolidin-2-one, known under the INN zolmitriptan, of formula (I):

The aforesaid patent describes the preparation of zolmitriptan by Fischer indole synthesis, using the corresponding phenyl hydrazine with an aldehyde. Said process nevertheless requires a stage of column purification of the end product, as well as the use of toxic reagents such as tin chloride for preparing the hydrazine, while it has an overall yield of only 18%.
Later, European patent EP 843672 describes optimised preparation of the intermediate (S)-4-(4-aminobenzyl)-1,3-oxazolidin-2-one in a one pot process (that is, without isolating the intermediates) and preparation of the zolmitriptan on the basis of this intermediate in a second one pot process which includes the formation of the diazonium salt of the intermediate, followed by the Fischer reaction (by addition of 4,4-diethoxy-N,N-dimethylbutilamine). However, this patent does not quote the yield o f zolmitriptan obtained. As a result, this applicant has carried out that procedure in order to reproduce and quantify it. The end product was obtained with yields of the order of 30% and with high impurity content due to the one pot reaction. It is therefore a process not applicable at industrial scale, either in terms of yield or of impurities.